The works just mentioned are concerned with the stars. But the
heavenly spaces contain nebulae as well as stars; and photography
can now be even more successful in picturing them than the stars.
A few years ago the late lamented Keeler, at the Lick Observatory,
undertook to see what could be done by pointing the Crossley
reflecting telescope at the sky and putting a sensitive
photographic plate in the focus. He was surprised to find that a
great number of nebulae, the existence of which had never before
been suspected, were impressed on the plate. Up to the present
time the positions of about 8000 of these objects have been
listed. Keeler found that there were probably 200,000 nebulae in
the heavens capable of being photographed with the Crossley
reflector. But the work of taking these photographs is so great,
and the number of reflecting telescopes which can be applied to it
so small, that no one has ventured to seriously commence it. It is
worthy of remark that only a very small fraction of these objects
which can be photographed are visible to the eye, even with the
most powerful telescope.
This demonstration of what the reflecting telescope can do may be
regarded as one of the most important discoveries of our time as
to the capabilities of astronomical instruments.
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